One of the main directions of the present development of high-efficiency capacitors with double electric layer is to make new electrode carbon materials with such a combination of properties as an optimal pore size, mechanical strength and high chemical purity.
Previously known are capacitors with a double electric layer (e.g., Japanese patent application No. 3-62296.1991), comprising two polarized electrodes divided by a separator, which are placed in a hermetic frame. The electrodes are made of active carbon and a binding agent, which consists of carbon black and ceramic powder. The electrode material has a porous structure, resulting in a specific electric capacitance not more than 25 F/cm.sup.3.
The deficiencies of such capacitors include: considerable leakage currents due to a great content of ash in the electrode material (3-8%); large variation in capacitance characteristics due to changes in micro porosity properties of the electrode material in the process of manufacture of the electrodes and the capacitor assembly; and the electrode material has low mechanical strength (this limits the use of these capacitors in constructions, which are working under conditions of high mechanical stress, e.g., vibrations).
Further, previously known (Japanese patent application No. 1-165108.1989r) are capacitors with double electric layer, comprising a frame of stainless steel; the frame comprises a bottom and a lid joined by a washer creating a hermetic container. In the frame, two polarized electrodes, saturated with electrolyte and separated by a porous separator, are situated. The electrodes are made of active carbon (80% mass) and a binding agent, which consists of ash (10% mass) and polytetrafluorethylene (10% mass). The material in the form of paste is applied to an electrically conductive underlayer and is then rolled and dried. From the resulting sheet product and the prescribed size electrodes are cut.
Such capacitors can operate over a wide range of temperatures. The electrode material provides the maximum specific electric capacitance within the limits of 20-25 F/cm.sup.3. However, these capacitors have all the deficiencies of the preceding ones.
For more than ten years new types of high energy capacitors have been developed based on very fine grained transmission metal oxides, like ruthenium oxide on a titanium foil, and these electrodes have been named super or ultra capacitors. High surface area carbons have also been used as electrode materials. Activated carbons with surface areas of a thousand or more square meters per gram are well known and results in super capacitors of coin size holding capacitances of several Farads.
For good performance as electrode materials it is desirable that these carbons high surface area. However, it is also important that a carbon electrode be stable in the electrolyte over the used capacitor voltage range, have high purity to minimize discharge or current leaks during long-time use and the specific electrical conductivity within the electrode must be high.